Over the course of the past few years the characteristics of Metropolitan Dense Wavelength Division Multiplexing (DWDM) Networks have evolved considerably. Just a few years ago a typical Metro DWDM system had 16×2.5 Gbit/s channels, 200 GHz spacing and un-amplified spans. Nowadays the average system supports 32×10 Gbit/s, 100 GHz spacing, optical amplification and chromatic dispersion compensation to cover distances of over 200 km. The next generation of Metro DWDM systems will raise the bar even further with respect to channel densities and distances, while continuing to reduce the footprint, the power consumption and increasing the flexibility of optical DWDM systems.
In the field of optical communications, Mach-Zehnder optical modulators are used to mix an RF information-bearing signal with a lightwave carrier by electromagnetic phase interferometry. Upon entering the modulator, the lightwave carrier is typically split into two signals that are coupled into separate waveguides formed in the crystal structure of the modulator. Electrodes are placed in close proximity to the waveguides in the device. An RF information-bearing signal is applied to the electrodes next to one of the waveguides. The propagation of the lightwave carrier through the crystal is affected by electric field variations that the RF signal causes. The electric field causes a local change in the refractive indices around the waveguides, thereby speeding up the propagation of the wave in one path while delaying the other. Thus, the relative phase of the two lightwave signals in the modulator is changed in proportion to the modulating signal applied to the electrodes.
At the output of the modulator the divided carrier signals are recombined. When the two signals having variations in relative phase caused by the RF input are recombined, phase interference occurs. Some of the interference is destructive and some constructive. This produces a modulated lightwave output having amplitude changes in proportion to the modulating RF signal. The modulated carrier can be coupled to a fiber optic medium for transmission over considerable distances.
An optical modulator, like its semiconductor counterparts in RF electronics, is a non-linear device. The typical Mach-Zehnder optical modulator comprises a lithium niobate (LiNbO3) crystal device having a non-linear modulation characteristic. To optimize the quality of the modulated output from an electro-optical modulator, it is desirable to apply a bias control to the device to set its operating point, or bias point, as close as possible to the center of its linear range.
Because the principle of operation of the Mach-Zehnder modulator is phase interferometry, the center bias point is very sensitive to temperature, input signal fluctuations, and manufacturing tolerances. Therefore, for good performance, it is desired to continuously monitor the output of the modulator and update the bias to ensure high dynamic range of the optical communications link. Conventional methods of moduator bias control for NRZ modulation do not work well with duobinary modulation.